In the paper industry, a coating material is deposited on the paper support in order to give the paper certain properties such as a smooth and hard-wearing surface.
Two main coating elements are used at present. One of the elements is constituted by composite blades forming a contact scraper for spreading the coating material moving relatively with respect to said blade. Another element is constituted by bars or rods for transferring coating material on to the paper support, said bars or rods being commonly referred to as rods in the technical sector under consideration.
The coating rods can have a smooth or fluted appearance on the surface.
When the coating rods are smooth, they serve to remove excess coating material when the latter is on the transfer roller before said coating material is transferred on to the paper support.
When the coating rods are fluted, they constitute a means of measuring out and transferring the coating material on to the paper support, owing to the fact that the coating rods go into a container of coating material so as to fill the flutes with said coating material before transferring it on to the paper support which passes over a roller.
These different techniques are well known to those skilled in the art.
As regards the use of coating rods, there are many problems to be solved.
A first problem to be solved concerns the surface condition which must be such that the transfer of coating material on to the paper must be carried out in a controlled and homogeneous manner, whatever the type of coating rod, smooth or fluted.
A second problem concerns the manufacture of the coating rod. This is because the current tendency is to use larger and larger coating rods, the length possibly reaching 10 meters and more.
A third problem concerns the production of the surfacing layer on the coating rod. When the surfacing layer is effected by thermal spraying, it is necessary to drive the coating rod rotationally during the entire deposition operation. However, one of the difficulties encountered is vibration of the coating rod which leads, for example, to bouncing of the particles which becomes greater as the vibration increases. One of the consequences of such bouncing of the particles sprayed on to the coating rod is the formation of a surfacing layer which is not uniform over the entire length of the coating rod.
A first solution advocated by the present invention is to hold the coating rod at its two ends on a frame and to center it with respect to the jaws of said frame. When the coating rod is very long, sagging at the center of the coating rod can occur. In order to reduce this sagging, it is possible to use known techniques which consist in using props positioned regularly over the length of the coating rod, the props generally being constituted by knurled wheels which are continuously being pushed by cylinders. But this is not satisfactory. This is because the knurled wheels, through their application on the coating rod, produce marks or impressions on said rod, such impressions creating surface defects at the time the surfacing layer is deposited. The more knurled wheels are used for propping up, the more there are impressions on the coating rod, the knurled wheels being separated from one another by a distance of around one meter. It is also necessary to note that a transfer of material may occur between the knurled wheels and the coating rod by friction and wear of said rod or said knurled wheels during the entire surfacing layer deposition operation.
Furthermore, and still concerning thermal deposition of the surfacing layer on the coating rod, it is necessary to take into consideration the expansion of the coating rod, created by the rise in temperature during said deposition, which expansion accentuating the sag at the center of the coating rod since the latter is held at its two ends on the frame.
The aim of the present invention is to remedy the aforementioned drawbacks and to propose a new coating rod and a new manufacturing method and a device for implementing the coating rod manufacturing method.
One object of the present invention is a method of manufacturing a coating rod, of the type consisting in holding the two ends of said rod on end supports of a frame, one of the ends being held fixed, and in driving said rod rotationally, and which is characterized in that it also consists in stretching said rod during the spraying of a surfacing layer on to said rod, the drawing out of said rod corresponding to a static elongation of around 0.2 to 0.001%.
According to another characteristic of the invention, the coating rod which is receiving a surfacing layer is held straight during the spraying of said surfacing layer.
According to another characteristic of the invention, the coating rod is made of ferromagnetic material and, more particularly, a chrome steel.
According to another characteristic, the device for implementing the method of manufacturing the coating rod is of the type comprising a frame on which there is mounted said coating rod, at least one motor rotationally driving, in a variable and adjustable manner, said coating rod, and means of thermal spraying of a surfacing layer, characterized in that it further comprises means of traction of one end of said rod, the other end being held fixed, said traction being performed during rotation of said coating rod and during the thermal spraying of said surfacing layer. The simultaneous control of the rotation and traction allows stabilization of the rod.
According to another characteristic of the present invention, the device comprises means of centering and holding at least part of the coating rod, said means being, for example, constituted by a bearing which may be magnetic, fluid or mechanical.
One advantage of the present invention lies in the fact that a perfectly stable and uniform coating rod is obtained, which allows an efficient removal of excess coating material before its transfer on to the paper support when it is smooth. In the case of fluted rods, the measuring out is performed better on account of the nature of the surfacing layer.
In all cases, the coating rod has a much longer lifetime which reduces the number of changes and therefore affords a considerable reduction in the cost of manufacturing the coated paper.